<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 80px;
      height: 30px;

      /* 短边的一半 */
      border-radius: 15px;

      background-color: #17f;
    }
  </style>
</head>

<body>
  <div></div>
</body>

</html>